THERMAL-BEHAVIOR OF POLYPROPYLENE FRACTIONS .2. THE MULTIPLE MELTING PEAKS

The appearance of double melting endotherms in dynamic d.s.c. runs of polypropylene fractions was studied with the aim of relating their app earance to the structure of the polymer. Fractions obtained by fractio nation either according to stereoregularity or to molecular weight wer e investigated. Double peaks were detected for low molecular weight, n on-isotactic fractions, as well as for low molecular weight fractions with isotacticities around 90%. Fractions consisted of pure alpha crys tal form and those showing double melting endotherms had low average i sotactic sequence lengths, generally below 80 propylene units. The occ urrence of double-peak shapes was attributed to two different processe s, one characteristic of highly stereoirregular material, and probably constituted by melting of two different crystalline species, and the other consisting of reorganization or recrystallization of less-ordere d domains of crystalline material taking place during the melting scan .